Natural and synthetic polymers have served essential needs in society. However, in recent times synthetic polymers have played an increasingly greater role, particularly since the beginning of the 20th century. Such synthetic polymers are commonly prepared by an addition polymerization mechanism, that is, free radical chain polymerization of unsaturated monomers. The majority of commercially significant processes are based on free-radical chemistry, or chain polymerization that is initiated by a reactive species, which often is a free radical. The source of the free radicals is termed an initiator or photoinitiator.
Photochemically induced polymerization reactions have become of great importance in industry, in particular for rapid curing of thin films, such as, for example, in the curing of paint coatings and plastic coatings on paper, wood, metal, and plastic or in the drying of printing inks. This curing by irradiation in the presence of photoinitiators is distinguished, compared with conventional methods for the drying or curing of coatings, by saving of materials and energy, low thermal stress of the substrate, and in particular a high curing rate. Moreover, the preparation of polymer materials by polymerization of the corresponding unsaturated monomeric starting materials is often carried out photochemically and by means of photoinitiators in such conventional processes as solution and emulsion polymerization. Since in the reactions mentioned, none of the reactants is usually capable of absorbing a sufficient amount of the photochemically active radiation, it is necessary to add photoinitiators.
Improvements in free radical chain polymerization have been focused both on the polymer being produced and the photoinitiator. Whether a particular unsaturated monomer can be converted to a polymer requires structural, thermodynamic, and kinetic feasibility. Even when all three properties are present, kinetic feasibility is achieved in many cases only with a specific type of photoinitiator. Moreover, the photoinitiator can have a significant effect on reaction rate, which, in turn, can determine the commercial success or failure of a particular polymerization process or product.
The primary function of a photoinitiator is to generate free radicals when the photoinitiator is irradiated with light of appropriate energy or wavelength. Photoinitiators are classified into “Type I” (or photocleavage) photoinitiators and “Type II” (or H-abstraction) photoinitiators according to the pathways by which the effective initiating radicals are generated.
In contrast to photocleavage photoinitiators that are decomposed by light directly into radicals that are effective in initiating polymerization, Type II photoinitiators require a hydrogen donor, or more generally a source of abstractable hydrogen's in order to generate radicals that are effective in initiating polymerization. The process of H-abstraction is usually a bimolecular reaction requiring the encounter of a photoinitiator and a hydrogen-donor. Any source of abstractable hydrogen's can be useful (for example, any structure that yields a stable radical after H-abstraction may serve as an “H donor”) and such sources include amines, thiols, unsaturated rubbers such as polybutadiene or polyisoprene, and alcohols.
Type I photoinitiators can generate free radical either of the two following mechanisms:
(1) the photoinitiator undergoes excitation by energy absorption with subsequent decomposition into one or more radicals, or
(2) a sensitizer molecule absorbs light and the excited sensitizer then transfers energy to the photoinitiator to generate free radicals.
The basic photochemistry and photophysics of both Type I and Type II photoinitiators have been well studied and utilized industrially in UV curable systems (see for example, Turro, N.J., Modern Molecular Photochemistry, 1991, University Science Books, chapters 7, 10, and 13.).
A number of Type I photoinitiators are commonly used in a variety of photocuring related applications and are commercially available. Among Type I photoinitiators, the hydroxyalkylphenone photoinitiators have proven to be particularly useful. Such photoinitiators include but are not limited to, benzoin ethers, benzil monoketals, dialkoxyacetophenones, hydroxyalkylphenones, and derivatives derived from these classes of compounds. α-Amino arylketones are also commonly used as Type I photoinitiators and are commercially available as are mono- and bis-acylphosphine oxides.
Most known photoinitiators (both Type I and II) have only moderate quantum yields (generally less than 0.5), indicating that the conversion of light radiation to radical formation needs to be made more efficient. The overall efficiency of photocuring process, in addition to overall composition of polymerizable material(s), depends on the quantum yield of radical generation of photoinitiator. To increase the overall efficiency of a photocuring, improvements in photoinitiators, as well as improvements in photoinitiating compositions, are necessary. In some cases, the commercial viability of certain systems can depend on whether a relatively modest improvement, for example, in the 2 to 10 times range, can be achieved. Improving photocuring efficiency is especially critical since with increasing diversification and specialization of processes and products in the area of coating techniques using polymer materials and, more and more frequent requirement of providing tailor-made solutions for these problems, increasingly requires higher and more specific demands on the photoinitiators and photoinitiating compositions. Therefore, in many cases, known photoinitiators do not fulfill, or at least not to an optimum degree, the demand made on them today. In most practical applications major, problems include the need to achieve even maximum (or theoretical) photoinitiator efficiency. These problems arise, for example:
(a) due to inefficient light absorption in pigmented systems,
(b) lack of compatibility with a wide range of binder systems and their reactive components and other modifying additives, or
(c) the storage instability in the dark of the systems containing the photoinitiator and the possible deterioration in the quality of the cured final product, such as yellowing, as a result of unconverted initiator residues and initiator degradation products.
Besides these challenges, there is an additional challenge of free radical photocuring inhibition by the presence of oxygen. Oxygen inhibition has always been a major problem for photocuring of acrylate-containing compositions containing multifunctional acrylate monomers or oligomers using a photoinitiated radical polymerization (for example, see Decker et al., Macromolecules 18 (1985) 1241.). Oxygen inhibition is due to the rapid reaction of carbon centered propagating radicals with oxygen molecules to yield peroxyl radicals. These peroxyl radicals are not as reactive towards carbon-carbon unsaturated double bonds and therefore do not initiate or participate in any photopolymerization reaction. Oxygen inhibition usually leads to premature chain termination, resulting in incomplete photocuring. Thus, many photocuring processes must be carried out in inert environments (for example, under nitrogen or argon), making such processes more expensive and difficult to use in industrial and laboratory settings.
Various methods have been proposed to overcome oxygen inhibition of photocuring:
(1) Amines that can undergo a rapid peroxidation reaction can be added to consume the dissolved oxygen. However, the presence of amines in acrylate-containing compositions can cause yellowing in the resulting photocured composition, create undesirable odors, and soften the cured composition because of chain transfer reactions. Moreover, the hydroperoxides thus formed will have a detrimental effect on the weathering resistance of the UV-cured composition.
(2) Dissolved oxygen can be converted into its excited singlet state by means of a red light irradiation in the presence of a dye sensitizer. The resulting 1O2 radical will be rapidly scavenged by a 1,3-diphenylisobenzofuran molecule to generate a compound (1,2-dibenzoylbenzene) that can work as a photoinitiator (Decker, Makromol. Chem. 180 (1979), p. 2027). However, the photocured composition can become colored, in spite of the photobleaching of the dye, prohibiting this technique for use in various products.
(3) The photoinitiator concentration can be increased to shorten the UV exposure during which atmospheric oxygen diffuses into the cured composition. This technique can also be used in combination with higher radiation intensities. Oxygen inhibition can further be reduced by using high intensity flashes that generate large concentrations of initiator radicals reacting with oxygen, but hydroperoxides are also formed.
(4) Free radical photopolymerization can be carried out under inert conditions (Wight, J Polym. Sci.: Polym. Lett. Ed. 16 (1978) 121), which is the most efficient way to overcome oxygen inhibition. Nitrogen is typically continuously used to flush the photopolymerizable composition during UV exposure. On an industrial UV-curing line, which cannot be made completely airtight, nitrogen losses can be significant, thus making the process expensive and inefficient. This is an even greater concern if argon is used to provide an inert environment.
Other less common ways of overcoming oxygen inhibition of acrylate photopolymerization include using a wax barrier and performing UV exposure under water. Each of these techniques has disadvantages that have made them less likely for commercial application.
The use of phosphites as antioxidants and stabilizers in polymeric compositions is known. For example U.S. Patent Application Publication 2004/0157949 A1 (Hu, [0058]) discloses the use of phosphites along with hindered amines and phenols as stabilizers in photocurable compositions. U.S. Patent Application Publication 2006/0173089 A1 (Jackson, [0026]) discloses the use of phosphites, hindered phenols, and hindered amines, antioxidant stabilizers in radiation-crosslinked polyolefin compositions. This publication also teaches that the use of some of these antioxidants in “excessive amount” can act as “radiation scavengers” and thus reduce effectiveness of photocuring. U.S. Patent Application Publication 2009/0292040 A1 (Sarmah) discloses the use of phosphites and hindered phenols as antioxidants in radiation curable liquid resins. None of these publications suggests that phosphites can be used with photoinitiators in photocurable compositions.
Phosphite stabilizers, for example, hindered neoalkyl phosphite compositions as disclosed in U.S. Pat. No. 5,464,889 (Mahood) exhibit undesirable odors, which make their handling and processing unpleasant and perhaps hazardous. Reducing the odors of phosphites would be an advance in the art for any use. It is clear from this discussion that in photopolymerization technology, there are continuing opportunities for improvements in free radical polymerization processes and photoinitiators. Moreover, there is a need in the art for new, energy-efficient photoinitiator compositions that can be used for use in a variety of polymerization and photocuring processes to prepare various articles in the presence of oxygen. The need for highly efficient photoinitiating compositions is particularly acute where absorption of light by the reaction medium may limit the amount of energy available for absorption by the photoinitiators. For example, in the preparation of color filter resists, highly pigmented resists are required for high color quality. With the increase in pigment content, the curing of color resists becomes more difficult. Hence, there is a need for photoinitiating compositions having a higher sensitivity and excellent resolution properties. In addition, there is a need for articles prepared from such photoinitiating compositions and articles containing such photoinitiator compositions to meet the industrial properties such as high solubility, thermal stability, and storage stability.